1. Field of the Invention
The present invention relates to a high breakdown voltage-semiconductor device the principal PN junction of which has a planar configuration.
2. Description of the Prior Art
On the basis of the structure of the PN junction, semiconductor devices are classified into the planar type, the mesa type and the bevel type. The planar type has the configuration in which the termination of each PN junction is exposed to one principal surface of a semiconductor substrate. The mesa type has the configuration in which the termination of a PN junction is exposed to an etched-down surface at the peripheral edge of one principal surface. The bevel type has the structure in which the termination of a PN junction is exposed to a side end surface connecting a pair of principal surfaces with each other, and in which the side end surface intersects with the PN junction at an acute angle in some cases and orthogonally in other cases.
The planar and mesa types are suitable for forming a number of semiconductor substrates from a semiconductor wafer having a large area and, accordingly, they are mainly applied to semiconductor devices of low power. On account of the handling, the bevel type is suitable for a power semiconductor device which has a semiconductor substrate of a large area as compared with those of the planar and mesa types. From the viewpoint of fabrication, the planar type requires only diffusion operations, while the mesa type additionally requires an etching operation. The bevel type involves the bringing of individual devices into the bevel shape. The order of ease of manufacture is the planar type, the mesa type and the bevel type.
On the other hand, since the surface of the semiconductor substrate to which the PN junction is exposed is active and is susceptible to the influence of the atmosphere, it is necessary to stabilize portions at and around the PN junction exposing surface by covering it with an insulator. The insulator covering is called a stabilizing film.
In the planar and mesa type semiconductor devices, the stabilizing film can be formed before dicing the large area semiconductor wafer into the many semiconductor substrates, and the formation of the stabilizing film is excellent.
In contrast thereto, in the bevel type of semiconductor device, the stabilizing film must be formed after dicing. In order to precisely form the stabilizing film on the diced small area semiconductor substrates, high precision techniques and considerable effort are required, so that manufacture of the bevel type is materially inferior to the manufacture of the planar and mesa types.
A discussion will now be presented with respect to the breakdown voltages of the respective types. The bevel type can readily achieve a high breakdown voltage in such a way that the angle of inclination of the side end surface at which the PN junction is exposed is so set as to make (positive bevel) the sectional area of a region of high impurity concentration large by which the region is parallel to the PN junction. Moreover, the angle of inclination is 15.degree. -60.degree. with respect to the PN junction, and the semiconductor substrate will not sharply become large-sized due to the inclination.
The mesa type has a lower in breakdown voltage than the bevel type, because the surface at which the PN junction is exposed has a negative bevel, that is, the inclined surface is such that the sectional area of a region on the high impurity concentration side relative to the PN junction by which the region is parallel to the PN junction becomes small. With the mesa type, a breakdown voltage of nearly 900 (V) is attained. In order to attain a higher breakdown voltage, the angle of inclination at which the PN junction is exposed must be made small. The negative bevel has the nature that, as the angle between the inclined surface and the PN junction or the angle of inclination decreases the breakdown voltage can be increased. For this reason, whereas in a thyristor, a PN junction of positive bevel and a PN junction of negative bevel are provided by inclining the side end surface, the inclination angle of positive bevel and that of negative bevel are different. For positive bevel the inclination is 15.degree. - 60.degree. as mentioned previously, while for negative bevel it is 1.degree. - 2.degree.. Accordingly, if a high breakdown voltage is intended for the mesa type, the angle of inclination must be nearly 1.degree. - 2.degree., which sharply enlarges the size of the semiconductor substrate with respect to conduction current. Also the breakdown voltage is subject to limitations.
In the planar type, the PN junction has a bent portion at which the field concentration arises. Since the impurity concentrations on both sides of the PN junction at the exposed termination of the PN junction are high, the spread of the depletion layer becomes small at the exposed PN junction. Due to these circumstances, it is difficult to obtain a semiconductor device having a high breakdown voltage. The breakdown voltage attained by the planar type is usually 300-400 volts. If a higher breakdown voltage is intended with the planar type, a region termed a field limiting ring must be formed at the periphery of the exposed PN junction part in a manner to be spaced from the PN junction and to surround it. Enhancement of the breakdown voltage accomplished by one field limiting ring is 300-400 volts, and the number of filed limiting rings is determined in accordance with the desired breakdown voltage. Accordingly, as the breakdown voltage becomes higher, the number of field limiting rings increases, which leads to the disadvantage that the semiconductor substrate sharply becomes large-sized with respect to conduction current.
The PN junction of the planar type is produced in such way that selective diffusion is performed using, as a mask, an oxide film formed on the semiconductor wafer surface. In this regard, it is difficult to form the oxide film perfectly free from pinholes, and portions which are not to be diffused are subjected to diffusion through the pinholes. Where diffusion through the pinholes takes place in the surface between the PN junction and the field limiting ring, the depletion layer does not spread to the field limiting ring and the portion of the diffusion through the pinhole breaks down, so that a desired breakdown voltage cannot be obtained. Further, in the planar type semiconductor device, the oxide film used for the diffusion mask is usually employed as the surface stabilizing film. Therefore, where pinholes exist in the oxide film, the semiconductor device is affected at the pinhole portions by the air and is prone to degrade the breakdown voltage.
In a planar type of semiconductor device which is resin-molded, there is the disadvantage that it is difficult to form a thick oxide film as the surface stabilizing film due to cracks and the necessary precision for mask registration and selective etching. In the device the breakdown voltage of which is 400-500 volts or more, the electric field extends even outside the oxide film due to the thinness of the film. Water having permeated through the resin is ionized by the electric field, and the ions adhere to the surface of the oxide film and create an induced channel in the vicinity of the surface of the semiconductor substrate. The induced channel becomes a cause of the leakage current in the device, and makes it impossible to attain a desired breakdown voltage.